This invention relates generally to mapping, and more particularly concerns method and apparatus to remotely determine the azimuthal direction of a probe which is inserted into a bore hole or well. In addition, it concerns method and apparatus to determine the probe's degree of tilt from vertical and to relate the latter to generated azimuth information. Further, the azimuth determining apparatus by itself, or in combination with the tilt measuring apparatus, may be housed in a carrier of sufficiently small diameter to permit insertion directly into available small I.D. drill tubing, thus eliminating the need to remove the tubing to enable such mapping.
In the past, the task of position mapping a well or bore hole for azimuth in addition to tilt has been excessively complicated, very expensive, and often inaccurate because of the difficulty in accommodating the size and special requirements of the available instrumentation. For example, magnetic compass devices typically require that the drill tubing be pulled from the hole and fitted with a length of non-magnetic tubing close to the drill head. The magnetic compass device is located within this non-magnetic section and the entire drill stem reassembled and run back in the hole as measurements are made. Thereafter, the magnetic compass instrumentation package must again be removed, requiring another round trip of the drill string. These devices are very inaccurate where drilling goes through magnetic materials, and are unusable where casing has been installed.
Directional or free gyroscopes are deployed much as the magnetic compass devices and function by attempting to remember a pre-set direction in space as they are run in the hole. Their ability to remember degrades with time and environmental exposure. Also, their accuracy is reduced as instrument size is reduced, as for example becomes necessary for small well bores. Further, the range of tilt and azimuthal variations over which they can be used is restricted by gimbal freedom which must be limited to prevent gimbal lock and consequent gyro tumbling.
One apparatus capable of solving the above problems is disclosed in U.S. Pat. No. 3,753,296, and involves use of a rate gyroscope; however, such a device requires high speed rotating of a rotor, as well as other working parts. In this regard, it is very desirable that the number of moving parts be reduced to a minimum, or eliminated, in order to optimize reliability and accuracy. Also, gyroscopes have temperature sensitivity, which becomes an important factor in bore hole drilling.